Equalization device for assembled battery

ABSTRACT

An equalization device for equalizing voltages of battery cells connected in series includes equalization switches, resistors, and a control circuit. Each equalization switch has energization terminals interposed between terminals of a corresponding battery cell. A current path between the energization terminals conducts when a control voltage not less than a threshold voltage is applied between control terminals of the equalization switch. Each resistor is connected between the control terminals of the corresponding equalization switch. The control circuit switches an equalization execution state and an equalization stop state in accordance with an equalization signal provided for each battery cell. In the execution state, the control circuit passes an electric current through the corresponding resistor to generate the control voltage not less than the threshold voltage. In the stop state, the control circuit causes the corresponding resistor to generate the control voltage less than the threshold voltage.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2013-181345filed on Aug. 2, 2013, the contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates to an equalization device for anassembled battery including multiple battery cells connected in series.

BACKGROUND

A battery, which is mounted on a motor-operated vehicle such as anelectric vehicle (EV) or a hybrid vehicle (HV) to supply electric powerto a motor of the vehicle, needs a high voltage of, for example, about300V. For this reason, the battery is configured as an assembled batteryincluding multiple battery cells, each of which has a cell voltage of afew volts, connected in series. A lithium ion battery cell, which hasbeen widely used in recent years, has a high cell voltage. Therefore,when the assembled battery is constructed with the lithium ion batterycells, the total number of battery cells in the assembled battery can bereduced, so that the size of the assembled battery can be reduced.

However, if each battery cell is not used within a predetermined cellvoltage range between its minimum effective voltage and its maximumeffective voltage, troubles such as a significant reduction in capacityof the battery cell and abnormal heat generation in the battery cell mayoccur. Further, if the battery cells have different cell voltages due tovariations in their capacity, an error of a voltage of the assembledbattery with respect to its target voltage may become large. For thisreason, an equalization device for monitoring voltages of battery cellsand equalizing the voltages has been demanded.

JP-A-2012-23848 corresponding to US 2013/0162213 discloses anequalization device having an equalization switch provided for eachbattery cell.

In addition to the equalization switches, the conventional equalizationdevice has a level shift circuit which is provided for each equalizationswitch and operates on a power supply voltage produced by voltages ofadjacent multiple battery cells. The level shift circuits areaccumulated from a low potential side to a high potential side. Theequalization device applies a drive voltage, which is level-shifted tothe high potential side, between control voltages of the equalizationswitch in accordance with an equalization signal inputted with respectto a ground potential, for example.

In the conventional equalization device, if a connector connecting theequalization device and the assembled battery is disconnected, or if apower supply voltage or a ground potential of a control circuit, whichreceives the equalization signal and supplies the equalization signal tothe level shift circuit, is lost, an operation of the level shiftcircuit becomes undefined, i.e., the drive voltage becomes undefined. Asa result, an operation of the equalization becomes undefined.

SUMMARY

In view of the above, it is an object of the present disclosure toprovide a an assembled battery equalization device capable of stablykeeping an equalization switch OFF even when a potential is undefined ora power supply voltage is lost in a control circuit which controls theequalization switch.

According to an aspect of the present disclosure, an equalization deviceis used for equalizing cell voltages of n battery cells of an assembledbattery, where n is a positive integer. The battery cells are connectedin series in such a manner that a first terminal of the (k+1)th batterycell is connected to a second terminal of the kth battery cell, where kis a positive integer less than n. The equalization device includesequalization switches, resistors, and a control circuit. Eachequalization switch is provided for a corresponding one of the batterycells. Each equalization switch has energization terminals, controlterminals, and a threshold voltage. A current path between theenergization terminals is interposed between the first terminal and thesecond terminal of the corresponding battery cell. The current pathconducts when a control voltage not less than the threshold voltage isapplied between the control terminals. Each resistor is connectedbetween the control terminals of a corresponding one of the equalizationswitches. The control circuit switches an equalization execution stateand an equalization stop state in accordance with an equalization signalprovided for each battery cell. In the equalization execution state, thecontrol circuit generates the control voltage not less than thethreshold voltage by passing an electric current through the resistorprovided for the battery cell corresponding to the equalization signal.In the equalization stop state, the control circuit causes the resistorprovided for the battery cell corresponding to the equalization signalto generate the control voltage less than the threshold voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentdisclosure will become more apparent from the following detaileddescription made with reference to the accompanying drawings. In thedrawings:

FIG. 1 is a schematic of an equalization system including equalizationdevice according to a first embodiment of the present disclosure;

FIG. 2 is a partial detailed view of the equalization device;

FIG. 3 is a state transition diagram of the equalization device;

FIG. 4 is a characteristic diagram of a lithium secondary battery cell;

FIGS. 5A, 5B, 5C, 5D, 5E, and 5F are schematics of a switch circuit anda constant current circuit according to a second embodiment of thepresent disclosure;

FIG. 6 is a schematic of a control circuit and its peripheral circuitaccording to a third embodiment of the present disclosure;

FIG. 7 is a schematic of a control circuit and its peripheral circuitaccording to a fourth embodiment of the present disclosure; and

FIG. 8 is a schematic of a control circuit and its peripheral circuitaccording to a fifth embodiment of the present disclosure.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described below with referenceto the drawings in which the same or similar number refers to the sameor similar part.

First Embodiment

A first embodiment of the present disclosure is described with.reference to FIGS. 1 to 4. An integrated circuit (IC) 11 shown in FIGS.1 and 2 is an equalization device for equalizing voltages of n batterycells BC1 to BCn of an assembled battery 12, where n is a positiveinteger. The assembled battery 12 is mounted on a motor-operatedvehicle, which has a motor and is capable of running by the motor, suchas an electric vehicle (EV) or a hybrid vehicle (HV). The assembledbattery 12 supplies electric power to the motor.

In the assembled battery 12, the battery cells BC1 to BCn are connectedin series in such a manner that a positive terminal (as a secondterminal) of the kth battery cell BCk (k=1, . . . , n−1) is connected toa negative terminal (as a first terminal) of the (k+1)th battery cellBCk+1. For example, according to the first embodiment, the assembledbattery 12 has eighty lithium secondary battery cells (i.e., n=80)connected in series, and each lithium secondary battery cell has a cellvoltage of 3.6V.

As shown in FIGS. 1 and 2, a Zener diode D1 is connected between thepositive and negative terminals of the battery cell BCi (i=1, . . . ,n). The negative terminal of the battery cell BCi is connected through aresistor R1 to a terminal Tim of the IC11. The positive terminal of thebattery cell BCi is connected through a resistor R2 to a terminal Tip ofthe IC11. A capacitor C1 is connected between the terminals Tim and Tip,When the voltages are equalized, the resistors R1 and R2 work to limit adischarge current and also work together with the capacitor C1 as afilter circuit.

The negative terminal of the battery cell BC1 is connected to areference potential. For example according to the first embodiment, thereference potential is a ground potential. A Zener diode D2 is connectedbetween the positive terminal of the battery cell BCn and the groundpotential. The positive terminal of the battery cell BCn is connectedthrough a resistor R3 to a power supply terminal Tp of the IC 11. Acapacitor C2 is connected between the power supply terminal Tp and theground potential. The resistor R3 and the capacitor C2 work together asa filter circuit. Inside the IC 11, the power supply terminal Tp isconnected to a power supply circuit (denoted as “PS” in FIG. 1) 15through a power supply line 13 and a switch 14. The power supply circuit15 produces a power supply voltage Vdd.

The IC 11 has an equalization switch (denoted as “ESW” in FIG. 1)provided for each of the battery cells BC1 to BCn. The equalizationswitch provided for each of a half of the battery cells BC1 to BCn is anN-channel MOS transistor, and the equalization switch provided for eachof the remaining half of the battery cells BC1 to BCn is a P-channel MOStransistor. Specifically, each of the battery cells BC1 to BCn/2 locatedon the low potential side is provided with an N-channel MOS transistorN1 as the equalization switch, and each of the battery cells BCn/2+1 toBCn located on the high potential side is provided with a P-channel MOStransistor P1 as the equalization switch. The drain and source of thetransistor N1, P1 corresponds to energization terminals, and the gateand source of the transistor N1, P1 correspond to control terminals. Asshown in FIGS. 1 and 2, a current path between the energizationterminals of the transistor N1, P is interposed between the positive andnegative terminals of the corresponding battery cell. As described indetail later, the current path conducts when a control voltage not lessthan a threshold voltage of the transistor N1, P is applied between thecontrol terminals of the transistor N1, P1.

A resistor R4 is connected between the gate and the source of thetransistor N1, P1. A series circuit of a switch circuit 16 and aconstant current circuit 17 is connected between the gate of thetransistor P1 and the ground potential. Likewise, the transistor N1 isprovided with a series circuit of the switch circuit 16 and the constantcurrent circuit 17. In the case of the transistor N1, in order to causean output current of the constant current circuit 17 to turn at thepower supply line 13 and to flow through the resistor R4, a resistor R5is connected between the power supply line 13 and the switch circuit 16,and a constant current circuit 18 and a P-channel MOS transistor Px areconnected between the power supply line 13 and the gate of thetransistor N1. Alternatively, a current mirror circuit can be used inorder to cause the output current of the constant current circuit 17 toturn.

A signal generation circuit 19 (denoted as “SG” in FIG. 1) receives anenable signal and an equalization signal from a microcomputer (denotedas “MIC” in FIG. 1) 20 which is located outside the IC 11. The enablesignal indicates whether an equalization process is enabled or disabled.The equalization signal indicates which battery cell BCi is to bedischarged and also indicates a discharge time during which theindicated battery cell BCi is to be discharged. During a period of timewhere the signal generation circuit 19 is supplied with the power supplyvoltage Vdd from the power supply circuit 15, the signal generationcircuit 19 outputs a current control signal to each switch circuit 16based on the enable signal and the equalization signal, therebyexecuting the equalization process to equalize the voltages of thebattery cells BC1 to BCn of the assembled battery 12.

The switch circuit 16, the constant current circuit 17, and the signalgeneration circuit 19 form a control circuit 21. When the currentcontrol signal is at an ON level (e.g., high level), the switch circuit16 is turned ON, and when the current control signal is at an OFF level(e.g., low level), the switch circuit 16 is turned OFF. When the switchcircuit 16 is turned ON by the current control signal of the ON level,an electric current flows through the resistor R4 so that thecorresponding transistor N1 or P1 provided for the battery cell BCi canbe turned ON. The IC 11 and the microcomputer 20 form an electroniccontrol unit (ECU) for monitoring the assembled battery 12.

The resistor R4 is connected between the drain and the source of anN-channel MOS transistor N2 in addition to between the gate and thesource of the transistor N1. Likewise, the resistor R4 is connectedbetween the drain and the source of a P-channel MOS transistor P2 inaddition to between the gate and the source of the transistor P1. Thetransistor N2, P2 is driven by a drive voltage outputted by a levelshift circuit (denoted as “LS”) 22. The level shift circuit 22 isprovided for each battery cell BCi and forms a drive circuit 23.

Each level shift circuit 22 operates on a power supply voltage producedby a series circuit of adjacent four battery cells including the batterycell BCi. For example, the level shift circuit 22 provided for thebattery cell BCn operates on cell voltages of four battery cells BCn-3,BCn-2, BCn-1, and BCn, and the level shift circuit 22 provided for thebattery cell BC1 operates on cell voltages of four battery cells BC1,BC2, BC3, and BC4.

Each level shift circuit 22 has a so-called cross-latch configuration.For example, as shown in FIG. 2, a series circuit of a P-channel MOStransistor P3 and an N-channel MOS transistor N3 is connected inparallel to a series circuit of a P-channel MOS transistor P4 and anN-channel MOS transistor N4 between a terminal Tnp and a terminal Tn-3 mof the IC 11. The gate of the transistor P3 is connected to the drain ofthe transistor P4, and the gate of the transistor P4 is connected to thedrain of the transistor P3.

The drains of the transistors P3 and N3 are connected together to thegate of the transistor P2 provided for the battery cell BCn to supplythe drive voltage. The drains of the transistors P4 and N4 are connectedtogether, and an inverted voltage of the drive voltage appears at thedrains of the transistors P4 and N4. The drive voltage outputted by thelevel shift circuit 22 provided for the battery cell BCn-1 adjacent tothe battery cell BCn on the low potential side is supplied as a drivesignal to the gate of the transistor N4. The inverted voltage of thedrive voltage outputted by the level shift circuit 22 provided for thebattery cell BCn-1 is supplied to the gate of the transistor N3.

The level shift circuits 22 provided for the battery cells BCn-1, BCn-2,BC2 are configured in the same manner as described above for the levelshift circuit 22 provided for the battery cell BCn. Thus, when thesignal generation circuit 19 outputs an ON-drive signal to the levelshift circuit 22 provided for the battery cell BC1, the ON-drive signalpropagates to the adjacent level shift circuit 22 in sequence, so thatall the transistors N2 and the P2 are turned ON at once. In contrast,when the signal generation circuit 19 outputs an OFF-drive signal to thelevel shift circuit 22 provided for the battery cell BC1, the OFF-drivesignal propagates to the adjacent level shift circuit 22 in sequence, sothat all the transistors N2 and the P2 are turned OFF at once.

Next, operations of the first embodiment are described below withfurther reference to FIGS. 3 and 4. The microcomputer 20 executes theequalization process for the assembled battery 12 at the right timingaccording to a state of a vehicle system. As shown in FIG. 3, when thevehicle system is in a normal mode or in an equalization mode, themicrocomputer 20 keeps a power supply (PS) signal at an ON level. Thenormal mode is a mode where an ignition (IG) switch of the vehicle is ONso that the assembled battery 12 can supply electric power to the motorof the vehicle. The equalization mode is a mode immediately after the IGswitch is turned OFF. When the PS signal is at the ON level, the switch14 of the IC 11 is turned ON so that the power supply voltage Vdd can begenerated. Thus, the internal circuitry of the IC 11 becomes operable.When the equalization mode ends, the vehicle system switches to astandby mode (i.e., dark-current mode) to save power consumption of theassembled battery 12. When the vehicle system is in the standby mode,the microcomputer 20 keeps the PS signal at an OFF level. When the PSsignal is at the OFF level, the switch 14 of the IC 11 is turned OFF sothat the power supply voltage Vdd can be lost.

In the normal mode and the standby mode, the enable signal transmittedfrom the microcomputer 20 to the IC 11 indicates that the equalizationprocess is disabled. At this time, the equalization signal transmittedfrom the microcomputer 20 to the IC 11 indicates no battery cell to bedischarged as denoted as “OFF” in FIG. 3. Thus, the IC 11 stops theequalization process in the normal mode and the standby mode. That is,in the normal mode and the standby mode, the IC 11 as the equalizationdevice is in an equalization stop state.

In the normal mode, the signal generation circuit 19 outputs the currentcontrol signal of the OFF level to the switch circuit 16 of each of allthe battery cells BC1 to BCn based on the enable signal. Thus, theswitch circuit 16 is turned OFF so that the output current of theconstant current circuit 17 cannot flow through the resistor R4. At thistime, since the gate-to-source voltage of the transistor N1, P1, as acontrol voltage of the equalization switch, becomes less than athreshold voltage Vth of the transistor N1, P1, the transistor N1, P1 isturned OFF. Thus, the resistor R4 can have a function to keep theequalization switch OFF.

Further, in the normal mode, the signal generation circuit 19 outputsthe ON-drive signal. At this time, the level shift circuit 22 of thedrive circuit 23 applies an ON-drive voltage between the gate and sourceof the transistor N2, P2. Thus, the transistors N2, P2 are turned ON atonce so that impedance between the gate and the source of the transistorN1, P1 can be reduced. Therefore, even when noise enters the IC 11, thegate-to-source voltage of the transistor N1, P1 remains less than thethreshold voltage Vth so that the transistor N1, P1 can be preventedfrom being accidentally turned ON.

In contrast, when the PS signal changes to the OFF level in the standbymode, the power supply voltage Vdd of the IC 11 is lost, so that theIC11 becomes undefined. That is, the level of the current control signaloutputted to the switch circuit 16 becomes undefined, and the level ofthe drive signal outputted to the level shift circuit 22 becomesundefined. Since the power supply voltage Vdd is lost, the currentcontrol signal is not kept at the ON level, As a result, the constantcurrent circuit 17 stops outputting the current, and no current flowsthrough the resistor R4. Thus, the resistor R4 clamps the potential ofthe transistor N1, P1 to its source potential, so that the transistorN1, P1 is kept OFF.

When the drive signal becomes undefined, the operation of the levelshift circuit 22 becomes undefined accordingly. However, although thetransistor N2, P2 which is OFF do not influence the equalizationprocess, the transistor N2, P2 which is ON can have a function to keepthe transistor N1, P1 OFF. That is, the drive circuit 23 and thetransistor N2, P2 have a function to turn OFF the transistor N1, P1, butdo not have a function to turn ON the transistor N1, P1. Therefore, inthe standby mode, the transistor N1, P1 stably remain OFF so that the IC11 can stably remain in the equalization stop state.

Although not shown in the drawings, the IC 11 detects the cell voltagesof the battery cells BC1 to BCn and transmits detection valuesindicative of the detected cell voltages to the microcomputer 20. Themicrocomputer 20 monitors based on the received detection values whetherthe cell voltages are equal to each other and fall within apredetermined voltage range (as a safe operation range). Themicrocomputer 20 identifies at least one battery cell whose cell voltageis higher than those of the other battery cells and needs to beequalized to those of the other battery cells. Further, themicrocomputer 20 determines a discharge time during which the identifiedbattery cell needs to be discharged in order to equalize the cellvoltage of the identified battery cell to those of the other batterycells. If the microcomputer 20 identifies multiple battery cells whosecell voltages are higher than those of the other battery cells, themicrocomputer 20 determines the discharge time for each of theidentified battery cells individually.

In the equalization mode, the microcomputer 20 transmits to the IC 11the enable signal indicating that the equalization process is enabledand the equalization signal indicating the identified battery cell to bedischarged and the discharge time during which the identified batterycell BCi is to be discharged. The signal generation circuit 19 executesthe equalization process based on the equalization signal. Thus, the IC11 is in an equalization execution state. Regarding a state of charge(SOC) and a cell voltage, a lithium secondary battery cell hascharacteristics shown in FIG. 4. In order to safely use the lithiumsecondary battery cell while increasing its life, it is necessary tocontrol the charge and discharge of the lithium secondary battery cellso that a cell voltage of the lithium secondary battery cell can fallwithin a safe operation range between its minimum effective voltage andits maximum effective voltage. The microcomputer 20 generates theequalization signal so that the cell voltage of the battery cell BCi canfall within the safe operation range.

The signal generation circuit 19 outputs the OFF-drive signal so thatthe transistors N2 and the P2 provided for the battery cells BC1 to BCncan be turned OFF at once. The signal generation circuit 19 outputs thecurrent control signal of the ON level to the switch circuit 16 providedfor the battery cell to be discharged while outputting the currentcontrol signal of the OFF level to the switch circuit 16 provided forthe battery cell not to be discharged.

The output current of the constant current circuit 17 flows through theresistor R4 when the switch circuit 16 is turned ON. At this time, thegate-to-source voltage of the transistor N1, P1, as a control voltage ofthe equalization switch, increases to not less than the thresholdvoltage Vth of the transistor N1, P1. Thus, the transistors N1, P1provided for the battery cell to be discharged are turned ON at once. Asa result, a charge current flows from the battery cell to be dischargedthrough the resistor R2, the transistor N1 or P1, and the resistor R1.Accordingly, the capacity of the battery cell to be dischargeddecreases, and the cell voltage of the battery cell to be dischargeddecreases. The signal generation circuit 19 outputs the current controlsignal of the OFF level to the switch circuit 16 provided for thebattery cell to be discharged when the individual discharge timeelapses.

As described above, according to the first embodiment, the IC 11 as theequalization device executes the equalization process for the assembledbattery 12 by means of a discharging control whenever the IG switch ofthe vehicle is turned OFF. Thus, it is possible to prevent a significantreduction in capacity of the assembled battery 12, abnormal heatgeneration in the assembled battery 12, and an error of an outputvoltage of the assembled battery 12 with respect to its target voltage.Further, when the assembled battery 12 is charged, the IC 11 can executethe equalization process for the assembled battery 12 by means of acharging control by turning ON the equalization switch provided for thebattery cell which do not need to be charged.

The transistor N1, P1 as the equalization switch is turned ON when itsgate voltage (as a control voltage) generated by the current flowingthrough the resistor R4 increases to not less than the threshold voltageVth. Thus, the transistors N1, N2 can be stably kept OFF underconditions where no current flows through the resistor R4, for example,when a connector connecting the IC11 and the assembled battery 12 isdisconnected, when the vehicle system changes to the standby mode sothat the power supply voltage Vdd of the IC 11 can be interrupted, etc.

In a conventional configuration where a control voltage of anequalization switch is generated by level-shifting, the number ofaccumulation of level shift circuits is larger as a battery cell is on ahigher potential side. As a result, a large layout size is required. Incontrast, according to the first embodiment, the control circuit 21 forgenerating the control voltage of the equalization switch by passing thecurrent through the resistor R4 is constructed with a series circuit ofthe switch circuit 16 and the constant current circuit 17. In such anapproach, the number of elements is reduced, and accordingly the layoutsize is reduced.

The transistor N2, P2 is connected between the gate and source of thetransistor N1, P1. When the transistor N2, P2 is turned ON by theON-drive voltage, the impedance between the gate and source of thetransistor N1, P1 is reduced. Thus, noise immunity is improved, so thata malfunction can be prevented. The transistors N2, P2 do not have thefunction to turn ON the equalization switches. Therefore, even when thedisconnection of the connector or the interruption of the power supplyvoltage Vdd occurs, it is ensured that the transistors N1, P1 are keptOFF.

The level shift circuit 22 is provided for each of the battery cells BC1to BCn. The level shift circuit 22 provided for the battery cell BC1receives the drive signal from the signal generation circuit 19 andtransmits the drive signal to the adjacent level shift circuit 22provided for the battery cell BC2 by level-shifting the drive signalwhile outputting the drive voltage to the transistor N2. In this way,since the drive signal propagates through all the level shift circuits22 in sequence, all the transistors N2, P2 can be turned ON and OFF atonce.

Second Embodiment

A second embodiment of the present disclosure is described below withreference to FIGS. 5A-5F. FIGS. 5A-5D show examples of the switchcircuit 16, and FIGS. 5E and 5F show examples of the constant currentcircuit 17. In the example of FIG. 5A, an N-channel MOS transistor isused as the switch circuit 16. In the example of FIG. 5B, an NPNtransistor is used as the switch circuit 16. In the example of FIG. 5C,although the N-channel MOS transistor is used as the switch circuit 16,the switch circuit 16 and the constant current circuit 17 are connectedin the opposite order to that shown in FIG. 5A. In the example of FIG.5D, although the NPN transistor is used as the switch circuit 16, theswitch circuit 16 and the constant current circuit 17 are connected inthe opposite order to that shown in FIG. 5B.

In the example shown in FIG. 5E, the constant current circuit 17 isconfigured so that an output current of a constant current source 24 canbe turned by a current mirror circuit constructed with NPN transistors25, 26. Alternatively, the transistors 25, 26 can be replaced withN-channel MOS transistors. The transistor 25 is connected in parallel tothe switch circuit 16. In FIG. 5E, the switch circuit 16 is an N-channelMOS transistor, and an inverted signal of the current control signal isinputted to the gate of the MOS transistor.

In the example shown in FIG. 5F, the constant current circuit 17includes an operational amplifier 27, an N-channel MOS transistor 28, aresistor 29, and a reference voltage circuit 30, and the switch circuit16 includes a first switch 16 a and a second switch 16 b. The firstswitch 16 a switches to the reference voltage circuit 30 side when thecurrent control signal is at the ON level, In contrast, the first switch16 a switches to the ground potential side when the current controlsignal is at the OFF level. The second switch 16 b is connected betweenan output terminal of the operational amplifier 27 and the groundpotential. In FIG. 5F, the second switch 16 b is an N-channel MOStransistor. When the current control signal is at the ON level, theconstant current circuit 17 outputs a constant current calculated bydividing a reference voltage of the reference voltage circuit 30 by aresistance of the resistor 29. The second switch 16 b is not essentialand can be omitted as needed.

Third Embodiment

A third embodiment of the present disclosure is described below withreference to FIG. 6. According to the third embodiment, the controlcircuit 21 includes a variable constant current circuit 31 instead ofthe switch circuit 16 and the constant current circuit 17 shown inFIG. 1. The variable constant current circuit 31 outputs a firstconstant current I1 when the current control signal is at the ON level,and outputs a second constant current I2 when the current control signalis at the OFF level. The second constant current I2 is sufficientlysmaller than the first constant current I1.

In this case, the following formulas (1) and (2) are satisfied.

I1×R4≧Vth  (1)

I2×R4<Vth  (2)

That is, the IC 11 is in the equalization execution state when thecurrent control signal is at the ON level in the equalization mode, andthe IC 11 is in the equalization stop state when the current controlsignal is at the OFF level in the equalization mode.

The other configurations, effects, and advantages of the thirdembodiments are the same as those of the first embodiment.

Fourth Embodiment

A fourth embodiment of the present disclosure is described below withreference to FIG. 7. According to the fourth embodiment, the controlcircuit 21 includes the constant current circuit 17, a resistor R6 as aresistance circuit, and a selection switch 32 instead of the switch 16.The selection switch 32 selects one of the constant current circuit 17and the resistor R6 according to the current control signal and connectsthe selected one in series to the resistor R4. Specifically, theselection switch 32 selects the constant current circuit 17 when thecurrent control signal is at the ON level, and selects the resistor R6when the current control signal is at the OFF level.

When the output current of the constant current circuit 17 flows throughthe resistor R4, the gate-to-source voltage of the transistor N1, P1increases to not less than its threshold voltage Vth, and the transistorN1, P1 is turned ON. A resistance of the resistor R6 is sufficientlylarger than a resistance of the resistor R4. In this case, the followingformula (3) is satisfied.

R4/(R4+R6)×V(Tnp)<Vth  (3)

In the formula (3), V(Tnp) represents a voltage of the terminal Tnp.

The other configurations, effects, and advantages of the fourthembodiments are the same as those of the first embodiment.

Fifth Embodiment

A fifth embodiment of the present disclosure is described below withreference to FIG. 6. According to the fifth embodiment, each of thebattery cells BC1 to BCn/2 located on the low potential side is providedwith a P-channel MOS transistor P1 as the equalization switch, and eachof the battery cells BCn/2+1-BCn located on the high potential side isprovided with an N-channel MOS transistor N1 as the equalization switch.

A series circuit of a switch circuit 16 and a constant current circuit17 is connected between the gate of the transistor N1 and the powersupply line 13. Although not shown in FIG. 8, the transistor P1 is alsoprovided with a series circuit of the switch circuit 16 and the constantcurrent circuit 17. Further, the transistor P1 is provided with acircuit for causing an output current of the constant current circuit 17to turn at the ground and to flow through the resistor R4.

The resistor R4 is connected between a drain and a source of anN-channel MOS transistor N2 in addition to between the gate and thesource of the transistor N1. Likewise, the resistor R4 is connectedbetween a drain and a source of a P-channel MOS transistor P2 inaddition to the gate and the source of the transistor P1. The, otherconfigurations, effects, and advantages of the fourth embodiments arethe same as those of the first embodiment.

Modifications

While the present disclosure has been described with reference toembodiments thereof, it is to be understood that the disclosure is notlimited to the embodiments. The present disclosure is intended to covervarious modifications and equivalent arrangements within the spirit andscope of the present disclosure.

The batteries cells (e.g., the battery cells BC4 to BCn-3) arranged inthe middle of the assembled battery 12 can be provided with either theN-channel MOS transistor N1 or the P-channel MOS transistor N1, Theequalization switch can be a bipolar transistor instead of a MOStransistor.

In the embodiments, a reference potential to which the negative terminalof the battery cell BC 1 is connected is the ground potential.Alternatively, the reference potential can be other than groundpotential. The transistor N2, P2 and the drive circuit 23 are not alwaysnecessary and can be omitted as needed. The power supply voltage onwhich the level shift circuit 22 provided for the battery cell BCioperates can be produced by a series circuit of adjacent two, three, orfive battery cells including the battery cell BCi.

The drive circuit 23 is not limited to the level shift circuit 22, aslong as the drive circuit 23 is configured so that the ON-drive voltagefor driving the transistor N2, P2 can be outputted in accordance withthe ON-drive signal. For example, the transistor N2, P2 can be driven inthe same manner as the equalization switch N1, P1 by connecting aresistor between the gate and source of the transistor N2, P2 and bypassing a current through the resistor using a series circuit of aswitch circuit and a constant current circuit.

A time at which the vehicle system enters the equalization mode, a timeat which the discharge starts in the equalization process, and a time atwhich the discharge ends in the equalization process are not limited tothose shown in FIG. 3.

The signal generation circuit 19 can restrict the equalization executionstate so that the cell voltage of the battery cell indicated by theequalization signal can be kept not less than the minimum effectivevoltage. For example, the signal generation circuit 19 can restrict theequalization execution state by stopping discharging the battery cellindicated by the equalization signal or reducing the discharging timeindicated by the equalization signal.

What is claimed is:
 1. An equalization device for equalizing cellvoltages of a plurality of battery cells of an assembled battery, thenumber of the plurality of battery cells being n which is a positiveinteger, the plurality of battery cells being connected in series insuch a manner that a first terminal of the k+1th battery cell isconnected to a second terminal of the kth battery cell, where k is apositive integer less than n, the equalization device comprising: aplurality of equalization switches, each equalization switch beingprovided for a corresponding one of the plurality of battery cells, eachequalization switch having energization terminals, control terminals,and a threshold voltage, a current path between the energizationterminals being interposed between the first terminal and the secondterminal of the corresponding battery cell, the current path conductingwhen a control voltage not less than the threshold voltage is appliedbetween the control terminals; a plurality of resistors, each resistorbeing connected between the control terminals of a corresponding one ofthe plurality of equalization switches; and a control circuit thatswitches an equalization execution state and an equalization stop statein accordance with an equalization signal provided for each batterycell, wherein in the equalization execution state, the control circuitgenerates the control voltage not less than the threshold voltage bypassing an electric current through the resistor provided for thebattery cell corresponding to the equalization signal, and in theequalization stop state, the control circuit causes the resistorprovided for the battery cell corresponding to the equalization signalto generate the control voltage less than the threshold voltage.
 2. Theequalization device according to claim 1, further comprising: aplurality of transistors, each transistor being provided for acorresponding one of the plurality of equalization switches, eachtransistor having energization terminals connected between the controlterminals of the corresponding equalization switch; and a drive circuitthat outputs an ON-drive voltage, wherein when the ON-drive voltage isapplied between control terminals of a first one of the plurality oftransistors, a voltage between the energization terminals of the firstone of the plurality of transistor becomes less than the thresholdvoltage.
 3. The equalization device according to claim 2, wherein thedrive circuit outputs the ON-drive voltage to all the plurality oftransistors at once in response to a single ON-drive signal.
 4. Theequalization device according to claim 3, wherein the drive circuitincludes a plurality of level shift circuits, each level shift circuitis provided for a corresponding one of the plurality of battery cellsand operates on a power supply voltage produced by a series circuit ofadjacent battery cells including the corresponding battery cell, a firstone of the plurality of level shift circuits outputs the ON-drivevoltage by level-shifting the ON-drive signal, and each of the others ofthe plurality of level shift circuits receives the ON-drive voltage, asthe ON-drive signal, from an adjacent level shift circuit.
 5. Theequalization device according to claim 1, wherein the control circuitincludes a plurality of series circuits, each series circuit is providedfor a corresponding one of the plurality of resistors and includes aconstant current circuit and a switch circuit, the constant currentcircuit outputs a constant current which causes the correspondingresistor to generate the control voltage not less than the thresholdvoltage, and the switch circuit is turned ON and OFF in accordance withthe equalization signal.
 6. The equalization device according to claim1, wherein the control circuit includes a plurality of variable constantcurrent circuits, each variable constant current circuit is provided fora corresponding one of the plurality of resistors, the constant currentcircuit selectively outputs one of a first current and a second currentin accordance with the equalization signal, the first current causes thecorresponding resistor to generate the control voltage not less than thethreshold voltage, and the second current causes the correspondingresistor to generate the control voltage less than the thresholdvoltage.
 7. The equalization device according to claim 1, wherein thecontrol circuit includes a plurality of constant current circuits, aplurality of resistor circuits, and a plurality of selectors, eachconstant current circuit outputs a constant current which causes thecorresponding resistor to generate the control voltage not less than thethreshold voltage, each resistor circuit causes the correspondingresistor to generate the control voltage less than the threshold voltagewhen being connected to the corresponding resistor, and each selectorselectively connects one of the corresponding constant current circuitand the corresponding resistor circuit in series to the correspondingresistor in accordance with the equalization signal.
 8. The equalizationdevice according to claim 1, wherein the control circuit restricts theequalization execution state so that the cell voltage of the batterycell corresponding to the equalization signal is kept not less than apredetermined minimum effective voltage.
 9. The equalization deviceaccording to claim 1, wherein the equalization signal is generated sothat the cell voltage of the battery cell corresponding to theequalization signal is kept not less than a predetermined minimumeffective voltage.